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<H3>group command 
</H3>
<P><B>Syntax:</B>
</P>
<PRE>group ID which style args 
</PRE>
<UL><LI>ID = user-defined name of the grid or surface group 

<LI>which = <I>grid</I> or <I>surf</I> 

<LI>style options for which = grid: <I>region</I> or <I>subtract</I> or <I>union</I> or <I>intersect</I> or <I>clear</I> 

<LI>style options for which = surf: <I>type</I> or <I>id</I> or <I>region</I> or <I>subtract</I> or <I>union</I> or <I>intersect</I> or <I>clear</I> 

<PRE>  <I>type</I> or <I>id</I> args
    args = list of one or more surface element types or IDs
      any entry in list can be a range formatted as A:B
      A = starting index, B = ending index
    args = logical value
      logical = "<" or "<=" or ">" or ">=" or "==" or "!="
      value = a surface element type or ID
    args = logical value1 value2
      logical = "<>"
      value1,value2 = surface element types or IDs
  <I>region</I> args = region-ID rflag
    region-ID = ID of region which grid cell or surface element must be in
    rflag = <I>one</I> or <I>all</I> or <I>center</I>
      <I>any</I> = one (or more) corner points of grid cell or surface element in region
      <I>all</I> = all corner points of grid cell or surface element in region
      <I>any</I> = center point of grid cell or surface element in region
  <I>subtract</I> args = two or more group IDs
  <I>union</I> args = one or more group IDs
  <I>intersect</I> args = two or more group IDs
  <I>clear</I> = no args 
</PRE>

</UL>
<P><B>Examples:</B>
</P>
<PRE>group sphere surf type 1 3
group sphere surf id 50 100:150
group sphere surf id <= 1000
group sphere surf id <> 50 250
group patch grid region leftedge
group patch surf region cutout
group boundary surf subtract all a2 a3
group boundary grid union lower upper
group boundary surf union lower upper
group boundary surf intersect upper leftside 
</PRE>
<P><B>Description:</B>
</P>
<P>Assign grid cells to grid groups or surface elements to surface
groups.  In SPARTA, a "grid group" is a collection of one or more grid
cells.  A "surface" group is a collection of one or more surface
elements (line segements in 2d, triangles in 3d).  Other commands take
group IDs as arguments so that they act on a set of grid cells or
surface elements.  For example, see the <A HREF = "compute_grid.html">compute
grid</A>, <A HREF = "compute_surf.html">compute surf</A>, <A HREF = "fix_ave_grid.html">fix
ave/grid</A>, <A HREF = "fix_ave_surf.html">fix ave/surf</A>, <A HREF = "dump.html">dump
grid</A>, or <A HREF = "dump.html">dump surf</A> commands.
</P>
<P>An individual grid cell can belong to multiple grid groups.  An
individual surface element can belong to multiple surface groups.
Each grid or surface group has a name which is specified as the <I>ID</I>
in this command.  Each grid group and surface group ID must be unique,
though the same ID can be used for both a grid and surface group.  IDs
can only contain alphanumeric characters and underscores.
</P>
<P>If the specified group ID already exists, grid cells or surface
elements are added to the group.  Otherwise a new group is created.
This means the group command can be used multiple times with the same
group ID to incrementally add grid cells or surface elements to the
group.
</P>
<P>A grid group with the ID <I>all</I> is pre-defined.  All grid cells belong
to this group.  Likewise, a surface group with the ID <I>all</I> is
pre-defined.  All surface elements belong to this group.
</P>
<P>After this command has performed its grid cell or surface elements
assignments, statistics about the group are printed to the screen, so
that you can check if the command operated as you expect.
</P>
<P>Note that this command assigns all flavors of child grid cells to
groups, which includes unsplit, cut, split, and sub cells.  See
<A HREF = "Section_howto.html#howto_8">Section 6.8</A> of the manual gives details
of how SPARTA defines child, unsplit, split, and sub cells.
</P>
<HR>

<P>The following styles can be used for grid groups.
</P>
<P>The <I>region</I> style puts all grid cells in the region volume associated
with the <I>region-ID</I> into the group.  See the <A HREF = "region.html">region</A>
command for details on what kind of geometric regions can be defined.
Note that the <I>side</I> option for the <A HREF = "region.html">region</A> command can
be used to define whether the inside or outside of the geometric
region is considered to be "in" the region.
</P>
<P>The <I>rflag</I> setting determines how a grid cell is judged to be in the
region or not.  For <I>rflag</I> = <I>one</I>, it is in the region if any of its
corner points (4 for 2d, 8 for 3d) is in the region.  For <I>rflag</I> =
<I>all</I>, all its corner points must be in the region.  For <I>rflag</I> =
<I>center</I>, the center point of the grid cell must be in the region.
</P>
<HR>

<P>The following styles can be used for surface groups.
</P>
<P>The <I>type</I> and <I>id</I> styles put all surface elements with the specified
types or surface element IDs into the group. These two styles can use
arguments specified in one of two formats.
</P>
<P>For surface elements, the "type" of each element is defined when the
elements are read from a surface file, via the
<A HREF = "read_surf.html">read_surf</A> command.  In the file, a positive integer
type value can be optionally defined for each element (default = 1).
The specified type values can also be incremented using the <I>typeadd</I>
keyword of the <A HREF = "read_surf.html">read_surf</A> commmand.
</P>
<P>For surface elements, the "ID" of each element is simply its index
from 1 to N, for all N surface elements that have been read in via the
<A HREF = "read_surf.html">read_surf</A> command.  The ordering of IDs is determined
by the order the elements appear in the read-in surface file.  If
multiple files are read (or the same file multiple times), IDs
increase monotonically each time new surface elements are added.
</P>
<P>The first format is a list of values (types or IDs).  For example, the
first command in the examples above puts all surface elements of type
1 and 3 into the group named sphere.  Each entry in the list can
optionally be a colon-separated range A:B, as in the second axample
above.  A "range" is a series of values (types or IDs).  The second
example with 100:150 adds all surface elements with IDs from 100 to
150 (inclusive) to the group named sphere, along with element 50 since
it also appears in the list of values.
</P>
<P>The second format is a logical operator followed by one or two values
(type or ID). The 7 valid logicals are listed above.  All the logicals
except "<>" take a single argument. The third example above adds all
surface elements with IDs from 1 to 1000 to the group named
sphere. The logical "<>" means "between" and takes 2 arguments. The
fourth example above adds all surface elements IDs from 50 to 250
(inclusive) to the group named sphere.
</P>
<P>The <I>region</I> style puts all surface elements in the region volume
associated with the <I>region-ID</I> into the group.  See the
<A HREF = "region.html">region</A> command for details on what kind of geometric
regions can be defined.  Note that the <I>side</I> option for the
<A HREF = "region.html">region</A> command can be used to define whether the inside
or outside of the geometric region is considered to be "in" the
region.
</P>
<P>The <I>rflag</I> setting determines how a surface element is judged to be
in the region or not.  For <I>rflag</I> = <I>one</I>, it is in the region if any
of its corner points (3 for triangle, 2 for line) is in the region.
For <I>rflag</I> = <I>all</I>, all its corner points must be in the region.  For
<I>rflag</I> = <I>center</I>, the center point of the line segment or centroid
point of the triangle must be in the region.
</P>
<HR>

<P>The following styles can be used for either grid or surface groups.
</P>
<P>The <I>subtract</I> style takes a list of two or more existing group names
as arguments.  All grid cells or surface elements that belong to the
1st group, but not to any of the other groups are added to the
specified group.
</P>
<P>The <I>union</I> style takes a list of one or more existing group names as
arguments.  All grid cells or surface elements that belong to any of
the listed groups are added to the specified group.
</P>
<P>The <I>intersect</I> style takes a list of two or more existing group names
as arguments.  Grid cells or surface elements that belong to every one
of the listed groups are added to the specified group.
</P>
<P>The <I>clear</I> style un-assigns all grid cells or surface elements that
were assigned to that group.  This is a way to empty a group before
adding more grid cells or surface elements to it.
</P>
<HR>

<P><B>Restrictions:</B>
</P>
<P>No more than 32 grid groups and no more than 32 surface groups can be
defined, including "all".
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "dump.html">dump</A>, <A HREF = "region.html">region</A>, <A HREF = "compute_grid.html">compute
grid</A>, <A HREF = "compute_surf.html">compute surf</A>
</P>
<P><B>Default:</B>
</P>
<P>All grid cells belong to the "all" grid group.  All surface elements
belong to the "all" surface group.
</P>
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